This invention relates generally to multi-channel jackets for microfilm, and more particularly to microfiche jackets adapted to accommodate thin microfilm strips as well as to a technique for manufacturing such jackets.
The Engelstein U.S. Pat. No. 3,238,655 entitled "Microfiche Master" discloses a microfiche master composed of a transparent jacket formed by two transparent plastic panels laminated together by ribs which are spaced to define a series of parallel channels or chambers adapted to accommodate microfilm strips. The loaded, multi-chambered jacket functions as a microfiche master from which reference copies may be made. This is effected by contact-printing through the front panel which is quite thin, the back panel being thicker to impart body to the jacket. Such microfiche masters are highly useful in storing and disseminating information.
The Engelstein patent points out that to facilitate contact-printing it is important not only that the top panel of the jacket be thin to minimize the separation between the sensitive film of the contact-printer and the microfilm strip in the jacket so as to obviate a loss of optical definition, but it is also essential to avoid any space between the film strip and the overlying top panel. Since this spacing is determined by the ribs which separate the top panel from the bottom panel of the jacket, the thickness of the ribs is made substantially equal to the thickness of the standard microfilm which is 5.5 mils. Thus the film strip is snugly received within the chamber.
The recent trend in microfilm technology is toward thinner films of 3 mils or less; for with a thinner film a greater footage of film may be carried on a standard reel, thereby enlarging the capacity of the microfilm system and reducing storage costs. But this creates certain problems in providing microfiche jackets suitable for thin microfilm; for then the ribs must be much thinner than those heretofore employed.
In the jacket disclosed in the Engelstein patent, preformed plastic or paper ribs are adhesively secured to the top and bottom panels. Hence the spacer ribs act as carriers for an adhesive agent to effect lamination. When the requirement is for very thin ribs to match very thin microfilm, it becomes commercially impracticable to manipulate the thin ribs in fabricating the jacket.
A known alternative approach to producing microfilm jackets without preformed ribs is to define the channels by an ultrasonic sealing technique wherein sealing is carried out along parallel lines, causing the top panel to belly out with respect to the bottom panel of the jacket, thereby defining the necessary channels or chambers for receiving the microfilm.
One drawback to this approach is that in ultrasonically sealing polyester for microfiche jackets, the heat created by the sealing action impairs the polymeric properties of the panel material and weakens the jacket, making it relatively easy to tear or fold the jacket along the weakened sealing lines. Moreover, with an established jacket format, the chambers defined by linear seals are significantly broader than the width of the film strips received therein so that the strips are loosely rather than snugly held.